Sputtering as a means to deposit thin films of a desired material on a substrate has become important in integrated circuits. In a sputtering system, material to be deposited on a substrate is removed from a sputter target by bombardment of the target with excited ions in the plasma state. Atoms or molecules of the target material are ejected from the target and are deposited on a workpiece or substrate. The ejection of these materials from the target is accompanied by significant heat build-up in the target area.
Normally, a sputtering system comprises a sputter source, a vacuum chamber, and means for positioning and holding the substrate in proximity to the sputter source. The sputter source normally comprises a target from which material is to be sputtered, means for holding the target in place, means for creating a plasma in the vicinity of the target, means for causing ions in the plasma to bombard the target, and means for cooling the target to prevent overheating.
Various means have been used in the past for holding sputter targets in place within the sputter sources. Such holding means must ensure that the target maintains good thermal contact with the cooling means so that heat generated in the target may be dissipated away.
In some sputter sources, circularly shaped targets are removably mounted within a fixed backing plate and are retained in place by various hold down features, such as clamps, springs, inserts, screws, etc. In these sputter source designs, the sputtering targets are independently removable from the fixed backing plates, for replacement of the targets due to target consumption, breakage, etc. Oftentimes, the target is soldered to the backing plate with the necessary requirement that the target be removed by melting the solder and separation of the target from the backing plate.
In order to cool the targets, cooling water is typically circulated alongside or through the backing plate positioned adjacent to the target. In some cases, the thermal contact between an outer rim of the target and the peripheral cooling wall of the backing plate is critical and is maintained by thermal expansion of the target against the cooling wall.
In other systems, when the target is replaced, both the spent target and the corresponding backing plate are discarded. The backing plate in such designs typically includes an upper annularly shaped planar surface adapted for flush receipt against the lower surface of the target. The backing plate further includes an annular ring extending downwardly from the planar surface having an outer mounting flange at the lower end of the annular ring. The target plate and backing plate may be joined via soldering, welding, brazing, or other metal bonding techniques.
Due to the differing coefficients of thermal expansion (“CTE”) of the target and backing plate members, the high heat levels attained by the target during high power sputtering result in excessive stress exerted along the target/backing plate interface. Warpage and differential shrinkage of both target and backing plate members may occur despite the cooling function, resulting in severely impeded cooling of the target with attendant target failure.